This invention relates to a circuit for reducing the magnitude of temperature-dependent variation in the voltage output of a band-gap voltage reference circuit.
The operating parameters of monolithic integrated circuits typically exhibit a temperature dependence. Among the sources of such temperature dependence are the base-emitter voltage drop (V.sub.BE) of a transistor, which has a negative temperature coefficient typically on the order of -2 mV/.degree.C., and the difference in the base-emitter voltage drops (.DELTA.V.sub.BE) of two mismatched transistors which, through the thermal voltage (V.sub.T), exhibits a positive temperature coefficient proportional to absolute temperature.
In the design of an analog integrated circuit such as a voltage regulator, it is necessary to establish a voltage or current reference within the circuit which is substantially independent of variations in temperature. A band-gap voltage reference circuit often is utilized to provide such a reference voltage or current. Such a circuit produces a relatively stable output voltage by compensating the negative temperature coefficient of a base-emitter voltage drop V.sub.BE with the positive temperature coefficient of a voltage difference .DELTA.V.sub.BE. More particularly, the two temperature coefficients are generated in the circuit and the positive temperature coefficient of voltage difference .DELTA.V.sub.BE due to thermal voltage V.sub.T is scaled with a temperature-independent scale factor (K) and combined with the negative temperature coefficient of base-emitter voltage drop V.sub.BE to obtain an output voltage with nominally zero temperature dependence.
In practice, however, the voltage output of a band-gap voltage reference circuit retains a degree of temperature dependence because the temperature coefficients of opposite polarity are both non-linear, such that the respective rates of drift vary with temperature. As a consequence, the two coefficients do not remain in a fixed proportional relationship as the temperature changes, and a nonlinear net temperature coefficient results. Further, the devices which make up the circuit typically exhibit other non-linear temperature coefficients which are not individually compensated. The sum of the uncompensated temperature coefficients produces a net non-linear variation in output voltage as the temperature changes.
For example, in one type of band-gap voltage reference circuit, known as a Brokaw Cell band-gap reference, the output voltage exhibits a temperature dependency which causes the output voltage to gradually fall off at lower and higher temperatures, giving an output voltage curve having the approximate shape of an inverted parabola when plotted against temperature. This degradation of output voltage at lower and higher temperatures limits the minimum temperature coefficient which can be obtained as temperature range increases.
Many circuits which utilize band-gap references also need over-temperature protection. Such protection is necessary to prevent a high-power circuit such as a voltage regulator from sustaining permanent damage due to excessive temperature rise caused by high power dissipation. A thermal shutdown circuit provides the necessary protection by sensing the circuit temperature and automatically shutting down the circuit when the temperature exceeds a predetermined threshold level. Because a regulator may operate at temperatures close to the desired shutdown temperature, the thermal overload protection must not interfere with normal circuit operation at temperatures close to the shutdown temperature. Simple thermal shutdown circuits typically have low thermal gain. As a consequence, regulators using these simple shutdown circuits must set shutdown temperature higher than would be desirable.
In view of the foregoing, it would be desirable to be able to provide a voltage reference circuit including a band-gap reference circuit which produces a voltage output having a smaller temperature dependency than that of the band gap reference circuit.
It would further be desirable to be able to provide a voltage reference circuit including a band-gap reference circuit which is also capable of rapidly shutting down surrounding circuitry when the operating temperature exceeds a predetermined threshold value.
In addition, it would be desirable to be able to improve the temperature independence of the reference voltage provided by a band-gap reference circuit and to provide thermal shutdown capability without adding greatly to the complexity of the circuit.